Zeolite A is a synthetic zeolite first described in U.S. Pat. No. 2,882,243 and having a composition, expressed in terms of mole ratios of oxides, in the range: EQU 1.0.+-.0.2M.sub.2/n O:Al.sub.2 O.sub.3 :1.85.+-.0.5SiO.sub.2 :yH.sub.2 O
where M is at least one of hydrogen, ammonium, metals in groups and I and II of the periodic table, and the transition metals of the periodic table, n is the valence of M, and y is any value up to about 6. Depending on the cation(s) M present in the composition, zeolite A may be employed as a 3 .ANG. (Na--K), 4 .ANG. (Na only) or 5 .ANG. (Na--Ca) molecular sieve in drying and n-isoparaffin separations in the petroleum and natural gas industries.
The 3 .ANG. molecular sieves, in particular, are used primarily as drying agents for natural gas, cracked gases, a wide range of petrochemical gas streams, as a drying agent in all kinds of refrigerant systems and other dessicant applications. A more complete list of usages is given by J. J. Collins (Chem. Engr. Progress, v. 64, p. 66, 1968). Zeolite 3 .ANG. usually contains sufficient large potassium cations within the structure to block the entry of all molecules except water. This requires the replacement of between 60 and 90% of the sodium cations normally present in zeolite A with potassium cations.
It is known to prepare, for example, zeolite E (U.S. Pat. No. 2,962,355), zeolite W (U.S. Pat. No. 3,012,853), zeolite L (3,216,789), faujasite molecular sieves (U.S. Pat. No. 4,175,059) and certain sodium-potassium zeolites (U.S. Pat. No. 3,374,058) by directly reacting selected proportions of Na.sub.2 O, K.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2. In preparing zeolite A or 3 .ANG. molecular sieves, however, the pure sodium form of zeolite A is first synthesized, followed by an ion exchange technique using potassium salt solutions in up to three stages to replace a portion of the sodium. These stages involve batch exchange procedures, filtration and washing prior to the crystallization step. See, for example, U.S. Pat. No. 4,160,011. Such a method is costly because of the number of unit operations required thereby. Each cation exchange step is followed by filtration or centrifugation, which involve polluting effluents, loss of product, time and equipment.